A ballast provides power to a lamp and regulates the current and/or power provided to the lamp. When a lamp (e.g. a fluorescent lamp) nears the end of its usable life or breaks, the resistance of the lamp increases as seen by the ballast. The increased resistance requires the ballast to output higher voltages in order to maintain the current or power transferred to the lamp. Thus, the ballast develops very high voltages (e.g., voltages in excess of 500 volts AC) as the resistance continues to increase. The high voltage poses an electrocution hazard to a technician who needs to replace the old lamp because the increased voltage increases the risk that the electricity will arc to earth ground through the technician as he attempts to replace the lamp. Therefore, some ballasts are equipped with a protection circuit (e.g., an end of lamp life circuit) that detects output voltages in excess of a threshold and shuts down the high voltage output of the ballast. These ballasts also have a circuit for detecting when a lamp has been replaced and restarting the high voltage output of the ballast in order to light the new lamp (e.g., by resetting the end of lamp life circuit).
A ballast may receive power from multiple sources. For example, ballasts used in commercial buildings commonly receive power from a utility line supply and from a battery. Such ballasts include a primary ballast for powering a lamp from the utility line supply and an battery powered ballast for powering the lamp from the battery. The primary ballast is configured to power two lamps in a single fixture while the battery powered ballast is configured to only power one of the lamps. Generally, the primary ballast is providing power to both lamps whenever utility line power is available to the ballast and the light is switched on, and the battery powered ballast powers one lamp from the battery when the light is switched on but there is no utility line power available to the ballast such as during a fire or utility power outage.
A problem can occur in these existing ballasts when switching power sources, for example, when switching from battery power to utility line power after an interruption in utility line power to the ballast. That is, when utility line power is restored to the ballast, the ballast switches operation from the battery powered ballast to the primary ballast by shutting down the battery powered ballast and reconfiguring the connection between the ballast and the lamps such that the lamps can be powered by the primary ballast instead of the battery powered ballast. However, due to timing delays in the primary and battery powered ballasts, the primary ballast may fail to properly power to the lamps due to timing delays in the primary and battery powered ballasts. That is, due to timing delays or rebound action in the switches that reconfigure the lamp to ballast connection, the end of lamp life circuit of the primary ballast may erroneously detect that the lamps are broken or end the end of their useful lives and shut down the output of the primary ballast. These timing delays may also cause other protection circuits to trigger.
To overcome these timing delays, one prior art solution disclosed in U.S. Pat. No. 6,339,296 is to delay supplying power from the utility line to the primary ballast for a period of 5 to 10 seconds in order to allow adequate time for the switches that determine the lamp to ballast connection to reconfigure the connection and other transients in the ballast to settle. Thus, when the primary ballast receives power, the end of lamp life circuit (and other protection circuits) is not falsely triggered because the lamps have been connected to the primary ballast output for a number of seconds. One problem with this solution is that delaying power to the primary ballast introduces a relatively long wait time when switching between power sources (e.g. when switching from battery power to utility line power).